Electric discharge machining power supply, and electric discharge machining method

ABSTRACT

A power supply ( 100 ) and a method for electric discharge machining by repeatedly providing a current pulse to a working gap ( 3 ) formed between a tool electrode ( 1 ) and a workpiece ( 2 ) includes a d.c. power source (E), first switching elements (Tr 1 -Trn) connected between the d.c. power source and the working gap, capacitors (C 1 -C 8 ) connected in parallel with the working gap, second switching elements ( 67 ) for controlling current flow from the capacitor to the working gap, a detector ( 50 ) for detecting start of an electric discharge, and a controller ( 20 ) for controlling the first switching elements and the second switching elements in response to the detector. Current is supplied from the d.c. power source through the first switching elements to the working gap only for a first time interval (τ ON) starting from the commencement of electric discharge, and a current is supplied from the capacitor through the second switching elements to the working gap for only a second time interval (T) is shorter than the first time interval, starting from the commencement of an electric discharge.

TECHNICAL FIELD

The present invention relates to an electric discharge machine formachining a workpiece by generating electric discharge in a working gapformed between a tool electrode and the workpiece. More particularly,the invention particularly relates to an electric discharge machinepower supply and method for repeatedly applying current pulses to theworking gap using a capacitor.

BACKGROUND OF THE INVENTION

When power is applied from a d.c. power source to the working gap, whichis a microscopic gap formed between a tool electrode of an electricdischarge machine and a conductive workpiece, the resistance ofdielectric fluid in the gap is reduced. As the insulationcharacteristics of the dielectric fluid break down, an electricdischarge occurs and the so-called pulse ON-time commences. During thecontrolled ON-time, electric discharge machining current flows throughthe working gap. As a result, workpiece material is evaporated ormelted. Upon completion of the ON time, application of power issuspended during a controlled OFF-time, during which the insulationproperties of the dielectric fluid is restored. In this way, currentpulses having a controlled ON-time and a controlled OFF-time arerepeatedly supplied to the working gap. A steep rising edge of thecurrent pulses waveform is known to contribute to improved machiningspeed.

FIG. 4 is a circuit diagram illustrating an electric discharge machinepower supply according to the related art. A workpiece 2 is arranged ina work tank (not illustrated) filled with dielectric fluid such askerosene. A tool electrode 1 is positioned so that a working gap 3 ofmicroscopic size is formed between the tool electrode and the workpiece2. N series combinations made up of switching transistors Tr1-Trn andcurrent limiting resistors R1-Rn are connected in parallel between ad.c. power source E and the working gap 3. In order to simplify thedrawing, only two series combinations are shown, the other seriescombinations have been omitted from the drawing for clarity. An on/offswitching operation of the switching transistors Tr1 to Tr2 iscontrolled by a gate pulse signal GP.

Electronic components such as a unit for generating the gateway signalGP, a d.c. power supply E, switching transistors Tr1-Trn, and currentlimiting resistors R1-Rn are normally housed in a cabinet. This type ofcabinet is invariably arranged at a physical distance from mechanicalsections such as a member for supporting the workpiece and a member forproviding relative allowing movement between the tool electrode andworkpiece. In order to electrically connect the d.c. power source E andthe working gap 3, a suitable conductor such as a coaxial cable CC isprovided between the cabinet and the mechanical sections.

The power supply of FIG. 4 further includes a capacitor C connected inparallel across the working gap 3. A switch SW is connected between thecapacitor C and the working gap 3, and it is possible to selectively usethe capacitor C. The electrostatic capacity of the capacitor can be setin a range of, for example 0.0068-1.6 μF. The combination of thecapacitor C and the switch SW is arranged as close as possible to theworking gap 3. The capacitor C and switch SW are housed, for example, ina small box attached to a side wall of the work tank. If at least one ofthe switching transistors Tr1-Trn is turned on with the switch SW in theclosed state, the capacitor C starts to charge. When the chargingvoltage of the capacitor C exceeds a certain value, an electricdischarge current I flows through the work gap 3. At the same time ascurrent is supplied from the capacitor C to the work gap 3, current isalso supplied from the d.c. source E through the switching transistor tothe working gap 3. A current pulse waveform supplied from the capacitorC to the working gap 3 is characterized by a steep rising edge. Thissteep rising edge improves the machining rate. The circuit of FIG. 4 isparticularly useful in cases such as where a copper electrode is used sothat the surface of a steel workpiece may be finished with a surfaceroughness of 3 μmRy or less, or where a copper tungsten electrode isused when machining a sintered hard metal workpiece.

The time taken for an electric discharge to commence after the switchingtransistors are turned on varies depending on the condition of theworking gap 3. As a result, there is the drawback that current suppliedfrom the capacitor C to the working gap is not constant. For example, anelectric discharge may commence in the working gap before there has beensufficient charging of the capacitator. Also, even if the switchingtransistors Tr1-Trn are off, it is possible for an unexpected dischargeto occur in the working gap 3 because of electric charge stored in thecapacitor C.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electric dischargemachine power supply that can control a single charge of current pulseenergy supplied from a capacitor to a working gap so as to beessentially constant.

Another object of the present invention is to provide an electricdischarge machine power supply and method that can sufficiently ensuresufficient time to enable capacitor charge to be stored. Additionalobjects of the invention will be set forth in the description whichfollows, and will in part become apparent to those skilled in the artupon practicing the invention.

In order to achieve the above and other objects, one aspect of presentinvention is directed to an electric discharge machine power supply andmethod which repeatedly provides a current pulse to a working gap formedbetween a tool electrode and a workpiece, the power supply comprising ad.c. power source, first switching elements connected between the d.c.power source and the working gap, a capacitor connected in parallel withthe working gap, second switching elements for controlling current flowfrom the capacitor to the working gap, a detector for detecting thestart of an electric discharge, and a controller for controlling thefirst switching elements and the second switching elements in responseto the detector so that current is supplied from the d.c. power sourcethrough the first switching elements to the working gap only for a firsttime interval from commencement of electric discharge, and current issupplied from the capacitor through the second switching elements to theworking gap for only a second time interval, shorter than the first timeinterval, starting from commencement of an electric discharge.

According to another aspect of the present invention, there is providedan electric discharge machining power supply by repeatedly providing acurrent pulse to a work gap formed between a tool electrode and aworkpiece; the power supply, comprising a d.c. power source, firstswitching elements connected between the d.c. power source and theworking gap, a first cable for directing current from the d.c. powersource through the first switching elements to the working gap, acapacitor connected in parallel with the working gap, a second cable fordirecting charging current from the d.c. power source to the capacitor,second switching elements for controlling charge current flowing fromthe capacitor to the working gap, and a controller for controlling thefirst switching elements and the second switching elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating an electric discharge machiningpower supply of the present invention;

FIG. 2 is a circuit diagram illustrating a correction current supplycircuit of FIG. 1;

FIG. 3A, FIG. 3B, FIG. 3C, FIG. 3D, and FIG. 3E are timing chartsillustrating operation of the electric discharge machining power supplyof FIG. 1.

FIG. 4 is a circuit diagram illustrating an electric discharge machiningpower supply according the related art.

PREFERRED EMBODIMENTS OF THE INVENTION

An electric discharge machine power supply according to a preferredembodiment of the present invention will now be described with referenceto FIG. 1 and FIG. 2. Reference numerals that are the same as those usedin FIG. 4 represent the same structural elements, and furtherdescription of these elements will be omitted.

An electric discharge machine power supply 100, similar to the powersupply of FIG. 4, comprises a d.c. power source E, and a switchingcircuit 10 made up of n current limiting resistors R1-Rn and switchingtransistors Tr1-Trn. A series combination of resistors 41 and 42 fordetecting a voltage being applied across a working gap 3 is connected inparallel across the working gap 3. A signal DS, representing a voltagedetected at a connection point 43 between the resistors 41 and 42, issupplied to an electric discharge detector 50. The electric dischargedetector 50 compares the voltage signal DS with a reference voltage, andif it is determined that an electric discharge has started at the gap 3,an impulse signal DP is supplied to a gate pulse generator 20. Acorrection current supply circuit 60, for increasing the steepness of arising edge of a waveform of an electric discharge current I that hasbeen turned into pulse, is provided in the vicinity of the working gap3.

As shown in FIG. 2, the correction current supply circuit 60 comprises acapacitor circuit 61 made up of a plurality of capacitors C1-C8 chargedby the d.c. power source E, and switches SW1-SW8 connected to thesecapacitors. The electrostatic capacitance of the capacitor circuit 61may be set in the range of, for example, 0.0068-1.6 μF. using theswitches SW1-SW8. Charging terminals 60A and 60B of the correctioncurrent supply circuit 60 are connected to the d.c. power source E by alow inductance coaxial cable CC, while output terminals 60C and 60D areconnected to the working gap 3. Charging current is supplied from thed.c. power source E through the current limiting resistor 63 and a diode64 to the capacitor circuit 61. Charging time is fixed by a timeconstant RC. For example, when assuming a 15Ω resistor R63 is used and a1.6 μF. capacitor is selected in the capacitor circuit 61, the chargingtime is about 24 μs. Reference numeral 65 represents an acceleratingcapacitor. If electric discharge starts at the working gap 3, currentflows from the capacitor circuit 61, through the diode 66 and theswitching transistor 67 to the working gap 3. In order to prevent thiscurrent from being undesirably affected by inductance, the capacitorcircuit 61 is arranged as close as possible to the working gap 3. Theon/off switching operation of the switching transistor 67 is controlledby a gate pulse signal GP2 supplied from the gate pulse generator 20.

Operation of the exemplary electric discharge machine power supply 100will now be described with reference to FIG. 3A, FIG. 3B, FIG. 3C, FIG.3D and FIG. 3E.

As shown in FIG. 3A, at time t1 when the gate pulse signal GP1 goes to ahigh or “H” level, voltage is applied across the working gap 3 from thed.c. power source E, as shown in FIG. 3D. As shown in 3B, at time t1 thegate pulse signal GP2 also goes to an “H” level. At time t2 whenelectric discharge starts at the working gap 3, current I flows throughthe working gap 3, as shown in FIG. 3E and FIG. 3D, the voltage DSfalls. The electric discharge detector 50 detects this voltage drop, andas shown in FIG. 3C, supplies a signal DP representing start of electricdischarge to the gate pulse generator 20. The gate pulse generator 20keeps the gate pulse signal GP1 at an “H” level from time t2 until timet4 when the set ON time τ ON expires. During the time τ ON, current iflows from the d.c. power source E through the switching circuit 10 tothe working gap 3, but, as shown in FIG. 3B, this current i risescomparatively slowly because of the current limiting resistors in theswitching circuit 10. In order to increase the relatively slow risingedge of current i, the gate pulse generator 20 keeps the gate pulsesignal GP2 at an “H” level from time t2 until time t3 when a set timeinterval T expires. During time interval T, current flowing from thecapacitor circuit 61 through the switching transistor 67 to the workinggap 3 is combined with the current i, producing a steeper rising edge inthe waveform of current I. The time interval T is preferably set to sucha value that current i reaches a peak value, in, for example, 5μs, andis shorter than the time interval τ ON. Since the time interval T is setin the gate pulse generator 20, energy provided from the capacitorcircuit 61 to the working gap 3 is constant during time interval τ ON.The gate pulse generator 20 maintains the gate pulse signal GP1 at an“L” level from time t4 until time t5 when set time interval τ OFFelapses. from time t6, when an electric discharge starts again, untiltime t7 when the time interval T elapses, current from the capacitorcircuit 61 flows to the working gap 3. From time t6 until time t8, whenthe time interval ON elapses, current I flows through the working gap 3.In this way, the gate pulse generator 20 controls the switchingtransistors Tr1-Trn and 67 so that the time interval τ ON, during whichthe workpiece 2 is machined by pulse current I, and the time interval τOFF during which the insulating characteristics of the dielectric fluidin the work gap 3 are restored, are constant. From time t3 until time t5the gate pulse generator 20 keeps the gate pulse signal GP2 at an “L”level, which means that flow of current from the capacitor circuit 61 tothe work gap 3 is prevented during the time interval τ OFF. Thecapacitor circuit 61 is charged from time t1-time t3, and ensures alonger charging time as compared with the capacitor C of FIG. 4.

It is not intended that the present invention be limited by the preciseforms disclosed, and obviously many modifications and variations arepossible in light of the above teaching. For example, the capacitorcircuit 61 may be is charged using the d.c. power source E, but it isalso possible for a separate d.c. power source to charge the capacitorcircuit 61. The illustrated embodiment was chosen and described in orderto best explain the principles of the invention and its practicalapplication. It is intended that the scope of the invention be definedby the claims appended hereto.

Translation for the drawings

FIG. 1

20 gate pulse generator

50 electric discharge detector

60 correction current supply circuit

What is claimed is:
 1. A power supply for repeatedly providing a currentpulse to a working gap formed between a tool electrode and a workpiece,comprising: a d.c. power source; a first switching element connectedbetween the d.c. power source and the working gap; a capacitor connectedin parallel with the working gap; a second switching element forcontrolling current flow from the capacitor to the working gap; adetector for detecting start of an electric discharge; and a controllerfor controlling the first switching element and the second switchingelement in response to the detector wherein current is supplied from thed.c. power source through the first switching element to the working gapfor a first time interval from commencement of an electric discharge,and current is supplied from the capacitor through the second switchingelement to the working gap for a second time interval, shorter than thefirst time interval, starting from commencement of the electricdischarge.
 2. The power supply according to claim 1, wherein thecapacitor and the second switching element are arranged in the vicinityof the working gap.
 3. The power supply according to claim 1, whereinthe capacitor is charged by the d.c. power source.
 4. The power supplyaccording to claim 1, wherein a current limiting resistor is connectedin series with the first switching element.
 5. A power supply forrepeatedly providing a current pulse to a working gap formed between atool electrode and a workpiece, comprising: a d.c. power source; a firstswitching element connected between the d.c. power source and theworking gap; a first cable for directing current from the d.c. powersource, through the first switching element, to the working gap; acapacitor connected in parallel with the working gap; a second cable fordirecting a charging current from the d.c. power source to thecapacitor; a second switching element for controlling charge currentflowing from the capacitor to the working gap; and a controller forcontrolling the first switching element and the second switchingelement.
 6. A method for machining a workpiece by repeatedly causingelectric discharges at a working gap formed between a tool electrode andthe workpiece, comprising the steps of: applying a voltage across theworking gap; detecting the start of an electric discharge at the workinggap; supplying current from a d.c. power source through a currentlimiting resistor to the working gap for a first time intervalcommencing with the start of an electric discharge; and supplyingcurrent from a capacitor to the working gap for a second time interval,shorter than the first time interval, commencing from the start of anelectric discharge.